Offline Diagnostics of Skin Sea Surface Temperature from a Prognostic Scheme and Its Application in Typhoon Forecasting Using the CMA-TRAMS Model over South China

In the Tropical Regional Atmospherical Model System of South China of the China Meteorological Administration (CMA-TRAMS), the skin sea surface temperature (T-s) remains fixed during the forecast time. This limits the model's performance in describing interactions between air and sea. The offline diagnostics and online analysis coupled with the CMA-TRAMS of T-s prognostic scheme were discussed. The results of the offline diagnostics showed that the profile shape parameter, v, and initial temperature, T-b, were sensitive to the forecasted T-s. Based on our observations, when v was set to 0.2 and T-b was the averaged T-s without obvious diurnal variation, the forecasted T-s was relatively reasonable. The forecasted T-s of CMA-TRAMS after coupling with the T-s scheme had diurnal variations during the overall forecast time, which was different from the fixed T-s from the uncoupled model. There existed a certain difference of forecasted T-s between uncoupled and coupled models in those days influenced by typhoons. The biases and Root Mean Square Errors (RMSEs) for the temperature and moisture in the lower layer and those for the wind speed in most layers were reduced and, therefore, the accuracy of environmental field forecasting was improved from the coupled model. The typhoon track errors after 36-h decreased due to the improvement of steering flow on the west side of subtropical high from the coupled model. However, the difference of typhoon intensity errors was insignificant, which might mean that the differences of forecasted T-s and heat flux between uncoupled and coupled model are small. The reasons for the small difference need to be further investigated.